Process for electrolytically colouring of aluminium which has previously been anodically oxidized

ABSTRACT

Aluminum is coloured by passing an alternating current between a previously anodized workpiece and a counter electrode in an aqueous acid tin salt.

United States Patent [19 Gedde [451 Nov. 19,1974

[ PROCESS FOR ELECTROLYTICALLY COLOURING 0F ALUMINIUM WHICH HAS PREVIOUSLY BEEN ANODICALLY OXIDIZED [76] Inventor:

24, 1713 Gralum, Sarpsborg, Norway [22] Filed: June 25, 1970 [21] Appl. No.: 49,951

[30] Foreign Application Priority Data UNITED STATES PATENTS 3,227,639 l/l966 Kampert 204/58 Ove Christopher Gedde, Gaupefaret 5/1968 Asada 204/58 3,382,160 3,472,744 10/1969 Kape 3,486,991 12/1969 Hatano et al. 204/58 FOREIGN PATENTS OR APPLICATIONS 741,753 11/1943 Germany 204/58 69,930 1/1946 Norway 204/58 1,715 3/1963 Japan 204/58 Primary Examiner-F. C. Edmundson Attorney, Agent, or Firm-Waters, Roditi, Schwartz and Nissen [57] ABSTRACT v Aluminum is coloured by passing an alternating current between a previously anodized workpiece and a counter electrode in an aqueous acid tin salt.

5 Claims, No Drawings PROCESS FOR ELECTROLYTICALLY COLOURING OF ALUMINIUM WHICH HAS PREVIOUSLY BEEN ANODICALLY. OXIDIZED The invention relates to a process for electrolytically colouring of aluminium or aluminium alloys which have previously been anodically oxidized. The coatings obtained by the process have a uniform colour of good durability.

A number of colouring methods have been suggested for aluminium and aluminium alloys which as a matter of ease will also be denoted as aluminium below. One of the mostly used methods consists in an immersion of anodically oxidized aluminium in a solution of organic dyes. By this immersion method some inorganic and semi-organic colouring solutions may also be used. An immersion of the previously oxidized aluminium into two successive inorganic baths in order to precipitate a coloured insoluble salt in the oxide layer is also known. In the first bath the oxidized aluminium is wetted with a reactant which is then reacted in another bath with a second reactant with formation of an insoluble coloured precipitate in the pores of the oxide layer.

Processes have also been developed based on a so called integrated colour-anodizing whereby the colour is developed by the formation of compounds due to reaction of alloying elements in the aluminium with the electrolyte, the compounds being precipitated in the alumina layer as it is formed.

However, the above-mentioned processes either yield coatings of low durability or coatings which are expensive to produce.

It has been attempted to overcome these disadvantages by using a two-step process based on the formation of an electrolytic deposit of coloured metal compounds in the previously formed alumina layer. By this process alternating current is conducted between an aluminium article and a counter electrode in an acid electrolyte containing suitable dissolved metal compounds. Thus, Norwegian patent No. 69930 relates to the use of an acid electrolyte containing metal salts from the group consisting of iron, cobalt, nickel, manganese and/or chromium salts together with small amounts of up to g/liter of another group comprising soluble compounds of arsenic, antimony, bismuth, selenium, tellurium and/or tin. With these composite electrolytes there are obtained various bronze tones on the aluminium article thereby that coloured compounds of the metals iron, cobalt, nickel and manganese can be electrolytically introduced into the pores of the oxide layer. The content of arsenic, antimony, bismuth, selenium, tellurium and/or tin must then not exceed a certain limit in order that a current density may be maintained which is utilizable in practice.

British patent No. 1022927 relates to'a similar process wherein an alternating current is passed between a previously anodically oxidized aluminium article, in order to colour the aluminium article, and a counter electrode of carbon, carborundum or aluminium in an aqueous bath containing a salt yielding a coloured metal oxide or metal hydroxide. As metal salts in the electrolyte it is suggested to use small amounts of a salt ofnickel, cobalt, chromium, cadmium, vanadium, gold, silver, iron or lead.

Although these last mentioned two-step processes offer a number of advantages compared with the immersion method and the integrated colour-anodizing stated above in that they yield e. g.a colour which is fast towards the influence of light, in that they are independent of the thickness and composition of the layer on the base material, in that they render it possible to obtain an exact dosage and in that they do not require any ion exchange material, these processes also suffer from a number'of serious disadvantages.

The degree of colouring is on the one hand dependent upon the current density during the second process step and on the other hand upon the duration during the second process step of the treatment in the bath of the aluminium article which has been anodically oxidized in the first process step. it has thereby been found that by the use of the known conventional electrolytes, e.g. the extended use today of nickel salts (cf. German patent No. 741753, Italian patent No. 338232, Canadian patents Nos. 762991 and 984928 and French patent No. 1477823), the current density must be kept as low as possible and it cannot'be increased beyond a certain value as puncture or the known concentric spalling off of the oxide layer will occur. In order to obtain a sufficient degree of colouring the treatment time must, therefore, be correspondingly increased, and this involves an uneconomic treatment. Further, the duration of the treatment with the alternating current is limited because even at low current densities a too long treatment time will also increase the risk of puncture. It is, accordingly, not automatically possible to increase the degree of colouring as desired.

On the other hand, due to the poor conductivity of the currently used electrolytes these lead to unfavourable scattering conditions, i.e. a non-uniform current distribution and, concomitantly an ugygn current aafiiry'wfirefiis dependent upon the shape of the treated aluminium article and which causes considerable variations in the degree of colouring at various places thereon. Localover-heating caused by the same reasons may also lead to discolourations.

' in order to at least partly avoid the detrimental con-' sequences of the above-stated'properties of the hitherto used electrolyte s ft has been suggested touse a large number of addition agents for the electrolytes. However, this makes the, two-step process substantially more difficult to carry out-and more expensive, preventing the desiredeconomical'operation and safety from being obtained.

it has now-surprisingly beenfound that the abovestated disadvantages may be avoided bypassing an alternating current between a previously anodically oxidized aluminium article and a counter electrode immersed in an electrolyte which contains essentially only tin ions as metal ions.

Thus, the invention relates to a process for electrolytically colouring of aluminium by passing an alternating current, between a previously anodically oxidized aluminium article and a counterelectrode immersed in an acid, aqueous solution of a metal salt, and the process is characterized in that as metal salt a tin salt is used.

The colours obtained by the present process have essentially bronze tones of varying degrees.

The counter electrode used in the present process is not decisive, however, it should preferably be made of tin if it is desired continuously to replace the tin ions in the electrolyte which are consumed during the colouring. or of a material which is not attacked by the electrolyte, e.g., stainless steel or titanium, in order to prevent contaminations from being introduced into the electrolyte. A counter electrode of aluminium may also be used. The counter electrode used in the present process consists preferably of tin or titanium.

By the use of tin salts according to the invention the current density may be strongly increased compared with all other known processes without any risk of puncture arising. A doubling of the current density compared with the conventional values is possible without difficulties, and even by a 3 times increase of the current density compared with the permissible values for other known electrolytes for obtaining bronze tones there was not observed any concentric spalling off of the oxide layer. Because it is possible in the present process to use a substantially higher current density very dark colour tones are obtained in a relatively short time, and there may e.g. economically and without risk of puncture be obtained a black colour by the use of electrolytes of this invention which are also suited for the production of other colour tones which are acceptable in practice.

ln addition, by the use of solutions of tin salts electrolytes are obtained having a particularly good scattering power, whereby the degree of colouring at the various places on thetreated aluminium article is substantially independent of its shape. This may essentially be referred back to the electrical properties and higher movability of the tin ions.

As tin salts it has been found particularly advantageous to use stannous chloride and stannous sulphate, and as acid in the electrolyte it has been found advantageous to use hydrochloric acid and sulphuric acid. However, other soluble tin salts and acids may also be used.

Despite the great advantages which are connected with the use ofa solution ofa tin salt in the present twostep process for colouring of previously anodically oxidized aluminium articles, it has hitherto not been suggested to use solutions of tin salts for this purpose. To be true, in some cases it has been suggested to add minor amounts of tin compounds to the hitherto conventional acid electrolytes of iron, cobalt, nickel, manganese or chromium salts whereby the electric scattering power of the electrolyte should be favourably influenced. However, as stated above the amount of these additions of tin compounds had to be restricted to very small amounts in order to avoid an unfavourable influence upon the effect of the electrolytes which are determining for the properties of the electrolytic bath. Electrolytic baths which specifically contain tin salts as the compounds to be determining for the properties of the electrolytic bath and the electrolytic colouring have hitherto not been suggested at all'for the second, coloring step of such two-step processes as stated above. This might essentially be due to the low stability of the usable tin salts. which can be referred back to the tendency of tin to be converted into tetravalent tin by the loss of 2 electrons per atom of divalent tin and in this manner to make the previous electrolytic bath useless after a short time due to a corresponding reaction. Of course, this effect also occurs in connection with the electrolytic baths used in the present process and which essentially contain a tin salt as metal salt, whereby the electrolytic baths, in spite of their giving a very good colouring, are somewhat unstable.

According to an embodiment of the invention, a weak organic acid is, therefore, added as a complexforming compound to the acid electrolytic bath containing tin salt and which is used according to the invention. There is thereby obtained a stable divalent tin compound having a low content of free tin ions, and this ensures that the bath becomes stable. The organic acids may e.g. belong to the group consisting of phe nolic acids and sulphonic acids, whereby aromatic radicals attached to the group preferably may from the benzene group.

In order to increase the conductivity of the electrolyte tartaric acid or ammonium tartrate may further be added.

Besides the above stated advantages which are obtainable by the use of a tin salt solution as electrolyte, it is in this manner obtained that the solution becomes more stable, with the additional advantage that the permissible contamination of the bath by aluminium and sodium is strongly increased compared with the otherwise known maximum permissible limits and may be e.g. up to times the values for the hitherto known electrolytes. The electrolyte used in the present process is very little sensitive as regards the ratio between the surface of the counter electrode and the aluminium article to be treated and the distance and choice of material for the counter electrode. Due to the good scattering power of the electrolyte the geometrical shape of both the counter electrode and the treated aluminium article is essentially without significance. Neither does a spalling off of the layer respectively a puncture occur even if exceeding the already substantially higher current densities used in the present process compared with the hitherto permissible current densities for known electrolytes. As by the present process a substantially higher current density may be used and because no puncture occurs by a prolonged treatment, it is possible in an economic manner to obtain very dark bronze tones besides a completely black colour.

Further, it is a substantial advantage of the present process that for preparing the electrolyte the usual commercially available chemicals may simply be used, in contrast to what may be done in connection with the known processes where this is partly not possible.

The present process can be carried out by the use of tin salts at a concentration of 0.5 to 20 percent by weight, preferably about 2.0 percent by weight, based on the electrolyte. This can preferably have a pH of about 1.5. The alternating current can be sinusoidal or have another curve shape and a frequency of 10 to 500 periods per second, preferably technical alternating current of 50 periods per second, and a voltage of 2 to 50 V, preferably 6 V, and there may be used a current density of 0.2 to L0 A/dm, based on the aluminium article. However, this upper limit may be exceeded.

When the electrolytic bath contains sulphuric acid it has been found particularly advantageous when renewing the bath to amend it to a certain degree either by the addition of a certain part of a previously used bath or by direct addition of an amount of aluminium sulphate, e.g. to provide about 50 g aluminium sulphate per liter. However, a lower concentration is generally sufficient to ensure a satisfactory colouring of the aluminium article from the beginning. The presence-of boric acid is also useful.

In the ensuing examples all percentages are based on weight.

EXAMPLE I An aluminium article which had previously been anodically oxidized in a percent aqueous solution of sulphuric acid was electrically connected with a counter electrode of tin in an aqueous electrolyte containing 2 percent stannous chloride and 50 ml concentrated hydrochloric acid per liter. An alternating current at 5 to 8 volt was supplied to the electrodes at room temperature for a period varying from 5 to 15 minutes, and the current density used was varied from 0.2 to 0.8 A/dm Very attractive bronze tones were obtained on the aluminium articles dependent upon the duration of the supply of the alternating current. The bronze tones became deeper with increasing duration of the supply of alternating current. The scattering power of the electrolyte was very good as appeared from the very uniform colour of the coating.

EXAMPLE 2 An aluminium strip which had previously been anodically oxidized for 45 minutes in a 15 percent aqueous solution of sulphuric acid was electrolytically coloured with the use of an alternating current. a counter elec-' trode of stainless steel and an aqueous electrolyte containing 40 ml concentrated HCl per liter and 2.7 percent SnCl per liter. The alternating voltage supplied was at 3 volt and had a frequency of 50 periods per second giving a current density of 0.4 to 0.5 A/dm The colouring process lasted for 10 minutes. and a very attractive brownish black colour was obtained on the aluminium strip. The coating obtained again had a very uniform colour, and this shows that the electrolyte had a good scattering power.

Contrary to what could reasonably be expected, the aluminium articles were not damaged by the chlorine containing electrolyte for the periods .used, and the corrosion resistance of the coloured coatings was very good after rinsing the aluminium articles with water and sealing of the articles in known manner.

EXAMPLE 3 There was used an electrolyte consisting of an aqueous solution containing g stannous sulphate per liter, 10 ml concentrated sulphuric acid per liter and l0 ml phenol sulphonic acidas complex-forming compound per liter.

EXAMPLE 4 There was used an electrolyte consisting of an aqueous solution containing 20 g stannous sulphate per liter, 10 ml concentrated sulphuric acid per liter, 10 g boric acid per liter and 4 to 5 g of an aromatic carboxylic acid, eg sulphophthalic acid. per liter, the carboxylic acid acting as complex-forming compound together with the boric acid.

EXAMPLE 5 There was used an electrolyte consisting of an aqueous solution containing 20 g stannous sulphate per liter, 10 ml concentrated sulphuric acid per liter, 2 g ammonium tartrate per liter, 10 g boric acid per liter and 4 g of an aromatic carboxylic acid per liter. The addition of the ammonium tartrate improved the conductivity of the electrolyte.

In examples 3 to 5 the same process as describedin example 1 was otherwise used.

It should further be remarked that during the second step for colouring the oxide layer formeddu'ring the first step of the process only a alternating current is used in order to deposit the colouring compounds, whereby in contrast to galvanic processes, no metallic tin is'of course deposited.

I claim: I

l. A process for electrolytically coloring an aluminum article which comprises first, anodically oxidizing said article to form an alumina layer thereon; immersing said article in an aqueous bath comprising a strong acid and a solution of about 0.5 to 20 percent by weight of a divalent tin salt, based on said bath, 'said tin salt being the predominant source of metal ions in said bath capable of forming and determining the color of the colored compounds in said alumina layer, and passing an alternating current between said anodically oxidized article and a counter-electrode via said bath to develop said color on said article. I

2. A process as claimed in claim 1, wherein said bath includes an aromatic sulfonic acid capable of complexing said tin in divalent form and selected from the group consisting of phenol sulfonic acids and sulfophthalic acid.

3. An electrolytic bath for the aluminum coloring process according to claim 1 comprising asolution of a stannous salt in a concentration of 1 percent to 20,

percent by weight, based on said bath, as the predominentmetallic coloring-component in a solution of a strong acid selected from the group consisting of sulfuric and hydrochloric acids.

4. The electrolytic bath according to claim 3 wherein said bath has the following composition:

Component Per Cent by Weight where indicated Stannous sulfate or chloride 1 to 20 Phenol sulfonic acids or sull'ophthalic 0 to l acid Ammonium tartrate or tartaric acid 0 to 0.2 Boric acid 0 to 1 Aluminum salt 0 to 5 Sulfuric or hydrochloric acid q.s. to pH of about l.5 Water q.s.

5. The process according to claim 1 wherein the tin salt is selected from the group consisting of stannous chloride and stannous sulfate. 

1. A PROCESS FOR ELECTROLYTICALLY COLORING AN ALUMINUM ARTICLE WHICH COMPRISES FIRST, ANODICALLY OXIDIZING SAID ARTICLE TO FORM AN ALUMINA LAYER THEREON; IMMERSING SAID ARTICLE IN AN AQUEOUS BATH COMPRISING A STRONG ACID AND A SOLUTION OF ABOUT 0.5 TO 20 PERCENT BY WEIGHT OF A DIVALENT TIN SALT, BASED ON SAID BATH, SAID TIN SALT BEING THE PREDOMINANT SOURCE OF METALS IONS IN SAID BATH CAPABLE OF FORMING AND DETERMINING THE COLOR OF THE COLORED COMPOUNDS IN SAID ALUMINA LAYER, AND PASSING AN ALTERNATING CURRENT BETWEEN SAID ANODICALLY OXIDIZED ARTICLE AND A COUNTER-ELECTRODE VIA SAID BATH TO DEVELOP SAID COLOR ON SAID ARTICLE.
 2. A process as claimed in claim 1, wherein said bath includes an aromatic sulfonic acid capable of complexing said tin in divalent form and selected from the group consisting of phenol sulfonic acids and sulfophthalic acid.
 3. An electrolytic bath for the aluminum coloring process according to claim 1 comprising a solution of a stannous salt in a concentration of 1 percent to 20 percent by weight, based on said bath, as the predominent metallic coloring-component in a solution of a strong acid selected from the group consisting of sulfuric and hydrochloric acids.
 4. The electrolytic bath according to claim 3 wherein said bath has the following composition:
 5. The process according to claim 1 wherein the tin salt is selected from the group consisting of stannous chloride and stannous sulfate. 